As is known in the art, air traffic control is a service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that aircraft do not operate in airspace where operations are prohibited; and assisting aircraft in distress. Orderly and expeditious flow assures the efficiency of aircraft operations along the routes selected by the operator. It is provided through the equitable allocation of resources to individual flights, generally on a first-come-first-served basis
As is also known, the need to easily and reliably identify aircraft led to the development of the so-called “identification friend or foe” (IFF) system which is known in non-military use as secondary surveillance radar (SSR) or (in the United States) as the air traffic control radar beacon system (ATCRBS). Both the civilian SSR and the military IFF systems are compatible such that military aircraft can safely operate in civil airspace. IFF and SSR systems generally contain a ground radar component often referred to as an interrogator which includes an antenna which is typically mechanically scanned (e.g. by rotating the antenna), but which can also be electronically scanned. The interrogator is often co-located with a primary radar. IFF/SSR systems also include a piece of equipment aboard the aircraft known as a transponder. The transponder is a radio receiver and transmitter which receives on one frequency (i.e. the interrogator frequency) and transmits on another. A target aircraft's transponder replies to signals from the interrogator by transmitting a coded reply signal containing the requested information.
IFF/SSR systems continuously transmit interrogation pulses (selectively rather than continuously in Mode-4, Mode-5, and Mode-S) as its antenna rotates, or is electronically scanned in space. A transponder on an aircraft that is within line-of-sight range ‘listens’ for the IFF/SSR interrogation signal and sends back a reply that provides aircraft information. IFF/SSR systems can transmit one of a plurality of different modes and the reply sent by the transponder depends upon the mode that was interrogated. Based upon the transponder reply, the aircraft is displayed as a tagged icon on a display of an air traffic controller, for example, at the calculated bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the air traffic controller without the benefit of IFF/SSR derived data.
As mentioned above, there are a plurality of different transponder modes. One particular mode is referred to as Mode 5 (M5). Mode 5 provides crypto secure capability similar to Mode S including transmission of automatic dependent surveillance-broadcast (ADS-B) and global positioning system (GPS) position (military only).
In areas of high multi-path conditions, IFF/SSR transponders sometimes reply to reflections of M5 interrogations (sometimes referred to simply as “M5 replies”). This can occur even when a primary waveform (i.e. the first waveform to be received by a transponder) contains a side lobe suppression pulse of proper amplitude to initiate suppression, if the reflected interrogation (i.e. the second received by the transponder) does not contain a sidelobe pulse of proper amplitude. This is because interrogation repeat protection occurs after decryption and an interrogation signal that is in side lobe suppression is never decrypted. This phenomena leads to many more M5 replies than is desired which in turn leads to more decryption loading on the interrogator and reduced reliability of target IDs in scenarios involving multiple aircraft.
One technique for solving the problem of transponders replying to multipath signals is to expand (i.e. increase) the acceptable position of a side lobe pulse to initiate suppression by two microseconds. This approach solves close-in multipath problems, but has many weaknesses. For example, this approach masks out good targets that happen to occur within the expanded window of a separate interrogator's side lobe pulse.